The present invention relates to amplifier systems and, in particular, to improving symmetry of such systems.
In many bipolar amplifier circuits, the maximum output current is usually limited by the base current of the amplifier output devices. These output devices are connected to the amplifier's output terminal and drive the amplifier's load. Therefore, the amplifier's load driving capabilities may be set by the capabilities of the output devices.
A typical amplifier output stage has asymmetric source and sink capabilities. The base current of one of the output devices may be provided by an integrator within the output stage or within the preceding gain stage. A typical non-complementary integrator can provide large currents to the base of one of the output devices, allowing the output device to pull strongly in its designated direction (i.e., it can sink or source a large output current). The base current of the other output device may be provided by a fixed current source, which limits the strength at which the output device can pull in its designated direction (i.e., it limits the maximum sink or source current of the device). Thus, the second output device may not pull as strongly as the first output device and it may not be able to deliver as large an output current as the first output device.
Multiple approaches have been used to address the asymmetry. One approach has been to employ a Darlington output transistor to increase the strength of the second output device by increasing the ratio of current in the collector terminal as compared to the base terminal of the output transistor, i.e., by increasing the current gain, beta, of the transistor. However, this approach compromises the amplifier's output voltage range, reducing it by approximately one VBE. A second approach uses a positive feedback circuit where a portion of the output transistor's collector current is fed back to the base terminal of the respective output transistor. However, due to the nature of using a positive feedback loop, an amplifier employing this technique may become unstable.
Accordingly, there is a need in the art for an amplifier system having improved symmetry in source/sink operations while providing stable operating conditions.